AMESBURY (CBS) – Phil Green wasn’t quite sure what he had, when he noticed the unusual rock on the banks of the Merrimack River.

His yard backs up to the river and he was on one of his frequent walks, looking for arrowheads. The tide was low, leaving behind exposed mud and smooth granite. And then he noticed something that just didn’t look right.

“There she was just sitting there, sticking up like that, and I said heck what is this,” recalls as he holds a large greenish colored rock. “It just didn’t belong.”

The rock was covered in mud when Phil found it. It was hard to see the burn marks on the side. At first he thought it was a rock used to make arrowheads. Then he suspected it might be meteorite. He used a metal detector to check and found it wasn’t metallic.

He suspected it might have come from outer space. But he had no idea just how unusual it actually was.

Phil was puzzled by the strange rock so he held on to it. But before long, he placed it in the yard and forgot about it. The rock sat under a tree for six years until a friend started asking questions.

Phil’s sister in law also thought it was from space so she sent it to a friend who works for NASA. That friend confirmed the rock was special, and that it wasn’t actually a rock at all.

What Phil had found was a piece of the Russian Space Station Mir. When Mir was de-commissioned, much of it burned up as it re-entered Earth’s orbit. The rest landed in the South Pacific Ocean. Somehow, one palm-sized chunk crashed into the Merrimack River in Amesbury.

Phil, who works as a custodian at Amesbury Elementary School, has brought it in as a teaching tool.

“I had a lot of fun taking it to school and showing it to all the kids,” he says.

Now it sits in his house, next to a letter from the NASA engineer.

The “rock” that started on Earth, went to space, and came back to Amesbury, now has a place as Phil’s prized possession.